With the ever-increasing demand for portable communication devices, reliability and efficiency of both user devices and devices in the supporting network has become of increasing importance. There are a number of different considerations, related to both individual elements as well as system elements, which affect these characteristics. For example, it is desirable to increase both bandwidth efficiency and power efficiency of a power amplifier in various communication devices. While bandwidth efficiency (the rate that data can be transmitted over a given bandwidth) is typically achieved using linear modulation, amplifier efficiency is a significant concern for achieving longer battery life and lower energy costs in transmitters as it usually dominates the power consumption in the system.
A transmitter of the portable communication device or of a communication device in the infrastructure (such as a base station) generally uses a radio frequency power amplifier (RFPA) as the final amplifying stage of a transmitter. The RFPA typically has a fixed power supply voltage. With a fixed supply voltage, however, the efficiency of the RFPA decreases as the output signal magnitude drops, leading to ineffectiveness and excessive peak power capability. To improve efficiency, it is desirable for the RFPA to continually operate near saturation, where the amplifier is close to or slightly gain compressed (about 0.5 dB below gain compression to 0.5 dB in gain compression). This can be achieved by modulating the power supply of the RFPA using the known technique of envelope tracking (i.e. the supply voltage of the RFPA tracks the output signal of the RFPA), which adjusts the power supply of the RFPA such that the power supply voltage of the RFPA follows the output signal thereby allowing the RFPA to continually operate near saturation.
While it is usually desirable in using power supply modulation for the RFPA to continually operate near saturation, a margin is provided to prevent “starvation” of the RFPA. The addition of margin to the power supply voltage is less than ideal because, it reduces average efficiency. Nevertheless, without providing some amount of margin, distortion in the amplified signal is often caused because of unexpected gain compression. Moreover, it is desirable for the RFPA to be adaptable to different forms of modulation, and thus the method of improving the efficiency of the RFPA to be able to operate for any given modulation. It is also desirable to be able to control the minimum voltage of the modulated supply, which improves efficiency especially when operating at low output power.
Therefore, a need exists for improved control of modulation of a power supply voltage to a power amplifier in order for the power amplifier to maintain high efficiency while operating linearly over a wide dynamic range. This control should also allow the power supply modulator implementation to be tolerant of design and component variations and to be backward compatible, thereby interfering minimally with core software used to implement a majority of the transmission functionality.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments shown so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Other elements, such as those known to one of skill in the art, may thus be present.